1. Field of the Invention
The present invention relates to bodies containing superabsorber polymers, methods of producing such bodies, and the use of such bodies, in particular for producing diapers, sanitary napkins, tampons and other articles of personal hygiene.
2. Description of Related Art
Absorbent structures, as are used for example in baby diapers, adult diapers and feminine hygiene, preferably contain so-called superabsorbers to increase the quantity of absorbable body fluid, make the diapers thinner and bind body fluids chemically within the developing gel body of the superabsorber. Leakage is thus prevented.
The SAP content in absorbent structures has been increased more and more over the past few years, with the following objectives: to reduce the thickness of the absorbent products, and to bind the body fluids in such a way that they are not released again even under mechanical stress (while being worn on the body, during movement, under pressure). Here the introduction of SAP in such products has proved successful because the fluid is bound to the SAP chemically, and not by capillary means. In this context, the fibers or fiber materials essentially have the function of transporting the fluid as rapidly as possible to the SAP, and the SAP itself has the function of receiving the fluid (reservoir function). It has been possible largely to avoid the problem of leakage and keep the skin dry by using special design features in the diaper, sanitary napkin, etc., primarily by backing the article with a thin non-fluid-permeable medium (a film) and further providing on the side toward the body a covering with an open structure, such as a thin, hydrophilic or hydrophilized nonwoven fabric or film with vacuum-expanded, three-dimensional perforations, and by using barrier leg cuffs and incorporating elastic elements. However, the advantage of the SAP--very high, pressure-resistant fluid uptake--is offset by the drawback of gel blocking and the associated slowing of fluid transport. Known means of enhancing gel stability, or endowing the SAP particles with a core/sheath structure in which the core is more tightly cross-linked than the sheath, result in a reduction of fluid absorption.
SAP (standing for superabsorber polymer) is known in a variety of embodiments, such as homophilic fibers composed 100% of SAP, bicomponent fibers having a core of nonabsorbent thermoplastic and a sheath of SAP, and finer and coarser powders.
SAP fibers blended with other fibers may be processed using known dry laying techniques to produce a fluid-absorbent nonwoven fabric. Although ease of processing argues in favor of such fibers made of SAP or containing an SAP component, one drawback is their relatively high price, particularly in comparison to fine and coarse powders or other particulates. An SAP staple fiber based on polyacrylic acid (or its sodium salt) costs approximately three times the price of SAP in powder form. For that reason, such fibers have not been able to achieve widespread acceptance, particularly for hygiene applications. Bicomponent fibers, in which only the sheath is made of SAP, have proved to be especially disadvantageous from the price standpoint. The sheath component constitutes only one-third to at most one-half of the total fiber mass. The absorption capacity is thus reduced accordingly and the cost of the fiber in relationship to absorbent effect is even less advantageous.
Using known dry compacting methods, such as mechanical interlacing or calendering in sheets or patterns, SAP fibers mixed with other fibers may be made into nonwoven fabrics, in which the risk of the SAP particles escaping is substantially reduced compared to SAP in other particulate forms. The SAP powder is added either by sprinkling onto a dense sheetlike body, such as tissue paper, or by dispersion within an open-pored, voluminous fiber structure. A loose deposition of fine SAP powder, or of nonfibrous SAP particles in other form, in an absorbent layer, is not advantageous. The position of the particles may change undesirably during use. In the worst case, the particles may escape from the absorbent product, if open-structured surfaces and/or open edges are present.
Numerous proposed solutions exist for fixing SAP particles to the fibers of the absorbent structures, or in other words, either within the sheetlike body or on the surface of the sheetlike body.
Diverse methods for moist pretreatment of the sheetlike body prior to the application of SAP powder, as well as dry (i.e., thermal) fixation methods, have been proposed in order to localize SAP particles on or within the absorbent article.
From European Patent A-0 719 531 it is known how to pretreat the support medium for the SAP with moisture/water. After the application or incorporation of the SAP particles, the water-soluble components of the SAP (inherent components) are activated to form a bond. After drying, the SAP particles are firmly bonded to the fibers of the absorbent layer.
In U.S. Pat. No. 3,070,095, a process is described in which SAP is sprinkled onto a tissue paper, then covered with a second tissue paper and compressed into a laminate by pressing between smooth rollers. This method, however, has the drawback that very little SAP can be applied, and the SAP is not penetrated with fibers and is thus only very weakly bonded. SAP powder escapes under the slightest mechanical stress. Moreover, such a liminate has an extreme tendency toward gel blocking, a characteristic of grave concern in hygiene applications.
WO 90/11181 and WO 91/10413 propose that fibers should be premoistened with an aqueous polymer dispersion instead of water. After drying, both the bonding forces of the synthetic polymer in the dispersion and the inherent bonding forces of the SAP particles come into play.
In all cases described thus far, the SAP particles are bonded to the fibers or incorporated into or onto the sheetlike body. But drying proves to be difficult and requires considerable outlay, because the water bound up in the SAP strongly resists re-release.
A great drawback of these SAP fixation methods is the fact that bonding is guaranteed only in the dry state. As soon as a gel body or hydrogel is formed from the SAP after the adsorption of fluid, the bonding force drops to zero or nearly zero.
European Patent A-0 720 488 describes a dry method for the fixation of SAP powder within a nonwoven fabric as the support medium for that powder. It proposes bonding fibers, either single-component (homophilic fibers) or bicomponent fibers having a higher-melting-point core and a lower-melting-point or lower-softening-point sheath. SAP is bonded to the fibers by fusion onto the bonding fibers, but in this case as well, the bonding force is almost entirely lost after the SAP is charged with fluid.
Solutions have also been proposed for giving fibers a coating of superabsorber. According to a method described in U.S. Pat. No. 4,721,647, a monomer, such as acrylic acid, methacrylic acid or vinylsulfonic acid, dissolved in water, is applied by drops to the fibers of an absorbent article, and is cross-linked there in the presence of initiators and cross-linking agents, to form a fiber coating of SAP. The concept is based on the fact that the SAP firmly envelops the fiber and is firmly bonded to the fiber even after swelling into a hydrogel-like state.
All methods described above for the application of SAP, via non-water-soluble precursors or olefin-unsaturated monomers, result in a substantial hardening in a nonwoven fabric, in comparison to the state of the fabric prior to the application of SAP, and at times cause a considerably lower application than when SAP powder is dispersed in the fabric.